Study on the Coupling Coordination between Ecological Environment and High-Quality Economic Development in Urban Agglomerations in the Middle Reaches of the Yangtze River

The ecological environment is the basis of high-quality economic development, and the coordinated development of the two is of great significance for promoting regional sustainable development. This study takes 31 cities in the middle reaches of the Yangtze River as samples, constructs an evaluation index system of the ecological environment (EE) and high-quality economic development (HQED), and uses a comprehensive evaluation method and a coupling coordination degree model to determine the development level, coupling coordination relationship, interaction coordination relationship and space–time evolution characteristics of the two. The results show that: (1) In the sample period, the overall level of EE and HQED increased simultaneously, but the two indexes of each city showed obvious heterogeneity. (2) EE and HQED have a coupling coordination relationship; the coupling degree (CD) is in a high coupling stage, and the coupling coordination degree (CCD) is in a good and moderate coordination state. (3) In the interactive coordination relationship, the CCD sequence of subsystems is coordinated development > shared development > innovative development > open development, and pressure subsystem > response subsystem > status subsystem. This study provides a new evaluation perspective for EE and HQED and puts forward suggestions for their coupling and coordinated development.


Introduction
With the excessive consumption of resources and the continuous pollution of the environment, the contradiction between global economic development and the ecological environment is intensifying [1]. Ecological environment (EE) protection and high-quality economic development (HQED) are important conditions for achieving regional sustainable development. China's economy is facing the urgent need for high-quality development. From the perspective of per capita GDP, China's per capita GDP will reach USD 10,500 in 2020, still lower than the USD 10,926 global per capita GDP. China is still a big developing country, and its economy is at a stage of high-speed growth toward high-quality development. China's 14th Five-Year Plan emphasizes "promoting high-quality development". Since it was published, HQED has become China's long-term strategic goal [2].
As far as EE evaluation research is concerned, there are not only the measurements of existing biological resources [3], but also research on environmental health and ecosystem vitality with drinking water safety, air quality, pollution discharge and other evaluation indicators [4]. The evaluation research on the quality of economic development includes building a comprehensive index system based on different spatial scales [5] and building an evaluation system from a certain dimension or index [6], as well as structural evaluation for the internal economy [7]. HQED evaluation and economic development evaluation focus 2 of 20 on different evaluation dimensions, including and expanding the content of the original economic evaluation and paying more attention to sustainability, coordination, efficiency and fairness [8]. HQED will be China's goal for a long period of time into the future and is of great concern to Chinese scholars. It is a high-quality development concept with Chinese characteristics and has been widely used in economic [9], education [10], tourism [11] and other related fields. With regard to the theoretical interpretation of the HQED concept, Ren et al. believes that high-quality development should be measured by coordination, innovation, sustainability, sharing and effectiveness [12]. Han et al. believes that highquality development is to achieve effectiveness in the economic and social aspects, and to innovate various social constructions from the perspective of coordinated development [13].
As far as the coupling and coordination research of the two is concerned, EE evaluation research has always been in a subordinate position to economic development research, rather than there being a mutual relationship between coupling and coordinated development [14]. Simon Kuznets put forward the Environmental Kuznets Curve (EKC) to describe the relationship between EE and economic development [15]. Many scholars have studied the income level and environmental quality of different countries and regions, and found various relationships such as U shape, M shape, N shape, etc. [16,17]. In addition, some scholars have tried to explore the coupling relationship between the two, such as the evolution of the integration of environmental protection and high-quality development of the industrial economy [18], and the study of urban economic development [19]. There has also been research on the coupling of economic growth and carbon emissions, etc. [20].
According to existing literature, the academic research on EE and HQED still has the following deficiencies: first, rich research results have been achieved on HQED, but the comprehensive evaluation and evaluation index system of EE and HQED are still insufficient. Second, there is only a small amount of literature on the coupling and coordination of EE and HQED. EE is the primary bottleneck constraining economic development [21] and is also the key factor for promoting HQED [22]. Therefore, the necessity of coupling and coordinated development of the two should be studied and discussed holistically in order to foster sustainable development of the regional economy. Specifically, in order to make up for the lack of existing research, this paper tries to solve the following three problems: (1) How to build an evaluation index system of EE and HQED and evaluate the comprehensive development level. (2) Determining whether there is a coupling coordination relationship between EE and HQED. (3) Identifying what kind of coupling and coordination relationship types and space-time characteristics exist in the sample space.
Based on the measurement of the EE and HQED levels of 31 cities in the urban agglomeration of the middle reaches of the Yangtze River in China from 2010 to 2019, this study examines the comprehensive development level, coupling and coordination relationship, and space-time evolution characteristics of EE and HQED. Compared with the existing research, the contributions of this study are as follows: (1) Based on the PSR model and the concept of high-quality development, a comprehensive evaluation index system of EE and HQED is built. (2) The urban agglomeration in the middle reaches of the Yangtze River is taken as the sample space, and the coupling coordination type, interaction coordination relationship and space-time evolution characteristics of EE and HQED are measured and evaluated. In particular, the analysis of the interaction and coordination relationship further explores the key factors affecting the coordinated development (3). This paper points out the problems and weaknesses of EE and HQED in the urban agglomeration in the middle reaches of the Yangtze River from a practical perspective and puts forward countermeasures and suggestions to promote the coupling and coordinated development of the two. The design and flow of this study are shown in Figure 1.

Study Area
The urban agglomeration in the middle reaches of the Yangtze River is located in central China. It covers 31 cities and an area of 326,100 square kilometers spanning the Hubei, Hunan and Jiangxi provinces, and is the largest urban agglomeration in China ( Figure 2) [23]. According to the China Statistical Yearbook, in 2020, the permanent population within the urban agglomeration in the middle reaches of the Yangtze River will account for about 9.4% of the country, and the GDP will account for about 9.3% of the country. The urban agglomeration in the middle reaches of the Yangtze River has unique geographical conditions. It faces the river and the sea and connects the east and the west, as well as the north and the south. It has a vast economic hinterland. It is in an important strategic position in China's construction of a pattern of major domestic circulation and dual domestic and international circulation [24].

Study Area
The urban agglomeration in the middle reaches of the Yangtze River is located in central China. It covers 31 cities and an area of 326,100 square kilometers spanning the Hubei, Hunan and Jiangxi provinces, and is the largest urban agglomeration in China ( Figure 2) [23]. According to the China Statistical Yearbook, in 2020, the permanent population within the urban agglomeration in the middle reaches of the Yangtze River will account for about 9.4% of the country, and the GDP will account for about 9.3% of the country. The urban agglomeration in the middle reaches of the Yangtze River has unique geographical conditions. It faces the river and the sea and connects the east and the west, as well as the north and the south. It has a vast economic hinterland. It is in an important strategic position in China's construction of a pattern of major domestic circulation and dual domestic and international circulation [24].

Study Area
The urban agglomeration in the middle reaches of the Yangtze River is located in central China. It covers 31 cities and an area of 326,100 square kilometers spanning the Hubei, Hunan and Jiangxi provinces, and is the largest urban agglomeration in China ( Figure 2) [23]. According to the China Statistical Yearbook, in 2020, the permanent population within the urban agglomeration in the middle reaches of the Yangtze River will account for about 9.4% of the country, and the GDP will account for about 9.3% of the country. The urban agglomeration in the middle reaches of the Yangtze River has unique geographical conditions. It faces the river and the sea and connects the east and the west, as well as the north and the south. It has a vast economic hinterland. It is in an important strategic position in China's construction of a pattern of major domestic circulation and dual domestic and international circulation [24].

Data Sources and Indicators
The data involved in this study include the China Regional Economic Statistical Yearbook The evaluation index system is the foundation to study the coupling and coordinated development of EE and HQED [25]. Based on the systematic review of relevant literature, this research builds an indicator system to comprehensively evaluate EE and HQED through comparative analysis and expert discussion. In terms of EE indicators, based on causal logic [26], this study deploys the PSR model (stress, state, response) to design the content of EE indicators. This is a common model for environmental quality assessment and is widely used in the design of EE-related indicators [27]. It complies with the index design principles of operability, representativeness and scientificity. Among them, the status subsystem includes the per capita green area and the green coverage rate of the built-up area. The pressure subsystem includes smoke (powder) dust emission per unit of industrial added value and exhaust gas emission per unit of industrial added value. The response subsystem includes domestic waste treatment rate and sewage treatment rate. Furthermore, compared with existing studies, some parameters have not been included in the indicator system, such as the per capita total grain [28], per capita industrial sulfur dioxide emissions [29], etc., due to the impact of changes in statistical indicators, data classification adjustment, statistical continuity and other factors.
HQED indicators are combined with the concept of high-quality development to build the HQED evaluation index system, using indicators from the four subsystems of innovative development, coordinated development, open development and shared development ( Table 1). Different from previous studies that focus on the quantitative characteristics of economic development [30], the evaluation dimension of HQED can more comprehensively reflect the sustainability [31], fairness [32] and stability [33] of economic development. Among them, innovation is the driving force of HQED. From the perspectives of innovation investment and innovation contribution, three indicators are selected to measure the innovative development level of the economy: the proportion of scientific research personnel expenditure in GDP, the number of scientific research personnel per 10,000 people, and the proportion of high-tech enterprises in the country. Coordination is the inherent requirement of high-quality development. Three indicators are selected to measure the level of coordinated development: the per capita GDP reflects the level of income coordination, the ratio of per capita consumption level to the national consumption level reflects the level of consumption coordination, and the proportion of the output value of the secondary and tertiary industries to the total output value reflects the characteristics of the industrial structure. Openness is the condition for the realization of HQED, and it is also the key link for the realization of HQED in the urban agglomeration in the middle reaches of the Yangtze River. The three indicators of foreign investment, foreign trade and tourism are selected to measure openness. Sharing is the fundamental pursuit of highquality development. Three indicators are selected, including the number of doctors per thousand people, the proportion of social security and employment expenditure in general public budget expenditure, and the registered unemployment rate, which specifically reflect the social welfare, social security and social stability.
(3) Calculation of comprehensive evaluation index.

Coupling Coordination Model
The coupling coordination model is used to analyze the coordinated development level of various factors [47]. In this study, coupling degree (CD) is used to reflect the degree of interaction between EE and HQED, and coupling coordination degree (CCD) is used to reflect the degree of coordination, especially the degree of benign coupling when the two interact.

Comprehensive Index Analysis
(1) From the perspective of a time series, the EE composite index and HQED composite index of the urban agglomeration in the middle reaches of the Yangtze River show a synchronous growth trend. With economic development, the EE has continuously improved. As shown in Figure  the two interact.

Comprehensive Index Analysis
(1) From the perspective of a time series, the EE composite index and H site index of the urban agglomeration in the middle reaches of the Yangtze synchronous growth trend. With economic development, the EE has con proved. As shown in Figure    (2) By calculating the average value of the EE index and HQED index of 31 cities from 2010 to 2019, and from the average value of the EE and HQED composite index of each city(see Figure 4), it was found that the EE index and HQED index of each city were significantly different. Of the 31 cities, only Wuhan and Changsha had a higher HQED index than EE index; of the two, Wuhan's HQED index (0.54) was significantly higher than the EE index (0.31) while Changsha's HQED index (0.39) was basically the same as the EE index (0.37). The EE and HQED of Ezhou and Huangshi were not much different. The HQED index of the other 27 cities was significantly lower than the EE index, especially for Ji'an, whose EE index was as high as 0.80, while the HQED index was only 0.07. The Fuzhou EE index was 0.78, significantly higher than its HQED index (0.04). For a long time, the emissions of pollutants from the economic development of cities have had adverse effects on EE, especially in heavy industrial cities such as Wuhan, where the development of industry has incurred serious environmental pollution problems [48]. In Ji'an, Fuzhou, Shangrao and other cities, the HQED index is at a low level among the 31 cities, and the EE index is at a high level. This phenomenon reflects that under a relatively low level of economic development, human damage to EE is relatively small [49]. the emissions of pollutants from the economic development of cities have had adverse effects on EE, especially in heavy industrial cities such as Wuhan, where the development of industry has incurred serious environmental pollution problems [48]. In Ji'an, Fuzhou, Shangrao and other cities, the HQED index is at a low level among the 31 cities, and the EE index is at a high level. This phenomenon reflects that under a relatively low level of economic development, human damage to EE is relatively small [49]. (3) From the perspective of spatial distribution, the overall level of EE has been continuously improved, and it can be divided into Jiangxi > Hubei > Hunan by province (2019). In order to show the spatial difference, the EE index was classified into high level (>0.  (3) From the perspective of spatial distribution, the overall level of EE has been continuously improved, and it can be divided into Jiangxi > Hubei > Hunan by province (2019). In order to show the spatial difference, the EE index was classified into high level (>0.7), medium level (0.5-0.7), low level (0.3-0.5) and very low level (<0.3). At the same time, the EE index of 2010, 2013, 2016 and 2019 was determined to draw the space-time change diagram of the EE comprehensive level ( Figure 5). Specifically, Ji'an, Fuzhou, Jiujiang and Yichang were at a high level in the four time-section datasets. The number of medium-sized cities is increasing. In 2010, only five cities, Jingdezhen, Shangrao, Xinyu, Yichun and Jingmen, were at the middle level. By 2013, there were eight medium-sized cities, and by 2019, there were 10 medium-sized cities. The number of lower-level cities increased from 15 in 2010 to 17 in 2019. In 2010, there were seven cities classified as lowlevel cities (Wuhan, Xiaogan, Tianmen, Qianjiang, Ezhou, Huanggang and Huangshi), all clustered around Wuhan. By 2013 and 2019, the EE level of these seven cities had risen, and all of them had been elevated to a classification of lower-level cities. The EE quality of Jiangxi Province has always been in the leading position in the central region. It has issued the guidance and action plan for the construction of ecological civilization, carried out a waste gas mine restoration project, and implemented wetland biodiversity protection, migratory bird habitat restoration, water ecological restoration and other protection and restoration projects focusing on the Poyang Lake basin [50]. and all of them had been elevated to a classification of lower-level cities. The EE quality of Jiangxi Province has always been in the leading position in the central region. It has issued the guidance and action plan for the construction of ecological civilization, carried out a waste gas mine restoration project, and implemented wetland biodiversity protection, migratory bird habitat restoration, water ecological restoration and other protection and restoration projects focusing on the Poyang Lake basin [50].  Tianmen is located in the Hanjiang Plain, with rich fishery and agricultural resources, and relatively weak secondary and tertiary industries. Therefore, the HQED level of the two places was lowered. Wuhan and Changsha are the provincial capitals of Hubei and Hunan, and they are also the core cities that drive the development of the urban agglomeration in the middle reaches of the Yangtze River. Relying on the original industrial base and convenient transportation location, they have been at the forefront of the economic development of the urban agglomeration in the middle reaches of the Yangtze River [51,52]. In recent years, the two cities have continuously strengthened industrial resources and technological innovation. For example, relying on the original automobile industry, Wuhan has focused on building new energy vehicles, cooperating with Changsha and Zhuzhou to promote the transformation of electric vehicles, and driving the Nanchang metropolitan area to develop new energy vehicles to maintain the advantages of special vehicles. At the same time, it has helped Yichun, Changde, Hengyang, Jiujiang, Jingzhou and other cities to build auto parts bases, promoted industrial cooperation within the urban agglomeration in the middle reaches of the Yangtze River, and optimized the regional layout of the industrial chain [53]. and convenient transportation location, they have been at the forefront of the economic development of the urban agglomeration in the middle reaches of the Yangtze River [51,52]. In recent years, the two cities have continuously strengthened industrial resources and technological innovation. For example, relying on the original automobile industry, Wuhan has focused on building new energy vehicles, cooperating with Changsha and Zhuzhou to promote the transformation of electric vehicles, and driving the Nanchang metropolitan area to develop new energy vehicles to maintain the advantages of special vehicles. At the same time, it has helped Yichun, Changde, Hengyang,Jiujiang, Jingzhou and other cities to build auto parts bases, promoted industrial cooperation within the urban agglomeration in the middle reaches of the Yangtze River, and optimized the regional layout of the industrial chain [53].

The Space-Time Evolution of CD
From the perspective of a time series, the CD changes of EE and HQED in the 31 cities over the past 10 years were mapped and the CD was graded, as shown in Tables 2 and 3. Overall, the CD of most cities in the urban agglomeration in the middle reaches of the Yangtze River was more than 0.7, which is at the advanced coupling level. In 2019, the proportion of cities with an advanced coupling level reached 84%, indicating that the EE and HQED of most cities in the urban agglomeration in the middle reaches of the Yangtze River formed a coupling relationship.  From the perspective of spatial distribution, the overall coupling level of EE and HQED in the urban agglomeration in the middle reaches of the Yangtze River has been continuously improved, with Hunan > Hubei > Jiangxi. In order to more accurately reflect the spatiotemporal changes in CD of the 31 cities in the urban agglomeration in the middle reaches of the Yangtze River, the CDs of 2010, 2013, 2016 and 2019 were determined for mapping (Figure 7). It can be clearly seen that Wuhan, Changsha and Nanchang, the central cities of the urban agglomeration in the middle reaches of the Yangtze River, were always in the advanced coupling stage. The running-in coupling was reduced and upgraded to advanced coupling for cities such as Yichang, Jingmen, Yiyang, Jiujiang, Shangrao, Jingzhou and Pingxiang. Antagonistic coupling decreased from four in 2010 to zero in 2019. In general, with the economic development and the deepening of the integration of the urban agglomeration in the middle reaches of the Yangtze River, the CD of EE and HQED also increased. In particular, Wuhan, Changsha and Nanchang, as the provincial capital cities, have always been in the advanced coupling stage due to their high level of economic development, relatively complete ecological protection measures, and the coupling relationship between EE and economic development that is mutually promoting, interdependent and closely coordinated.

The Coupling Coordination Relationship between EE and HQED
From the perspective of a time series, the CCD of EE and HQED in the 31 cities has improved continuously in the past 10 years, but it is still only at or below a good coordination level, and there is no city with high-quality coordination. CCD is classified as

The Coupling Coordination Relationship between EE and HQED
From the perspective of a time series, the CCD of EE and HQED in the 31 cities has improved continuously in the past 10 years, but it is still only at or below a good coordination level, and there is no city with high-quality coordination. CCD is classified as shown in Tables 4 and 5. The CCD level mainly includes high-quality coordination, good coordination, moderate coordination and low coordination.  From the perspective of a spatial distribution, the overall coordination level of EE and HQED in the urban agglomeration in the middle reaches of the Yangtze River is constantly improving, with Jiangxi > Hunan > Hubei. The CCD images of the 31 cities from 2010, 2013, 2016 and 2019 were captured for mapping (Figure 8). From the spatiotemporal changes of these CCDs, it can be clearly seen that the urban agglomeration in the middle reaches of the Yangtze River is mainly at the two coordination levels; namely, good coordination and moderate coordination. In 2019, there were 21 good-coordinated cities (accounting for 68%) and 10 moderately coordinated cities (accounting for 32%). Specifically, in 2010, the CCD of Tianmen, Xiantao, Qianjiang and Jingzhou cities was lower than 0.3, i.e., at the low-coordination level. In 2013, the number of low-coordination areas decreased by two, leaving only Xiantao (0.293) and Jingmen (0.288). In 2016, low-coordination areas continued to decrease, leaving only Tianmen (0.294). In 2019, there were no low-coordination cities, and other cities in Jiangxi, except for Pingxiang, rose to a good-coordination level. Espe-cially in Jiangxi Province, the CCD level of EE and HQED improved as a whole. Jiangxi Province is a major agricultural province in China with rich ecological resources and a good environmental foundation [54]. In recent years, Jiangxi has developed innovative infrastructure relying on the Poyang Lake Innovation Demonstration Zone and the science and technology innovation corridor on both sides of the Ganjiang River. It has development advantages in the national backbone of the internet direct connection points and new energy special vehicles, which has significantly improved the CCD level of EE and HQED.
Int. J. Environ. Res. Public Health 2023, 20, 3612 13 of 21 creased by two, leaving only Xiantao (0.293) and Jingmen (0.288). In 2016, low-coordination areas continued to decrease, leaving only Tianmen (0.294). In 2019, there were no lowcoordination cities, and other cities in Jiangxi, except for Pingxiang, rose to a good-coordination level. Especially in Jiangxi Province, the CCD level of EE and HQED improved as a whole. Jiangxi Province is a major agricultural province in China with rich ecological resources and a good environmental foundation [54]. In recent years, Jiangxi has developed innovative infrastructure relying on the Poyang Lake Innovation Demonstration Zone and the science and technology innovation corridor on both sides of the Ganjiang River. It has development advantages in the national backbone of the internet direct connection points and new energy special vehicles, which has significantly improved the CCD level of EE and HQED.

Coupling Coordination Relationship between EE and the HQED Subsystem
The coupling coordination relationship between EE and the four subsystems of HQED is shown in Figure 9.

Coupling Coordination Relationship between EE and the HQED Subsystem
The coupling coordination relationship between EE and the four subsystems of HQED is shown in Figure 9.

Coupling Coordination Relationship between HQED and the EE Subsystem
The coupling coordination relationship between HQED and the three subsystems of EE is shown in Figure 10. From the overall level of CCD, the CCD of the HQED and three subsystems show pressure subsystem > response subsystem > state subsystem (1). From the perspective of coordination with the pressure subsystem, the EE and the CCD of the response subsystem in Wuhan (0.994), Changsha (0.906), Ezhou (0.789), Nanchang (0.748) and Huangshi (0.731) were at a high-quality coordination in 2019. Huanggang (0.485) and Tianmen (0.411) were at a moderate level of coordination. The remaining 24 cities were at a good level of coordination. (2) According to the coordination relationship with the response subsystem, Wuhan (0.978), Changsha (0.901), Ezhou (0.780), Nanchang (0.732) and Huangshi (0.726) were at a high-quality coordination. Xiantao (0.467), Huanggang (0.454) and Tianmen (0.395) were at a moderate level of coordination. The remaining 23 cities were at a good level of coordination. (3) From the coordination relationship with the state subsystem, there were only two coordination levels: moderate coordination (18 cities) and good coordination (13 cities). It can be seen from the indicator system that the EE status subsystem was relatively stable but also vulnerable to damage. As shown in Figure 10a, the CCD of EE and status subsystems of cities in the middle reaches of the Yangtze River urban agglomeration were relatively stable and the gap was relatively small, indicating that the impact on EE status was relatively small in the development of the middle reaches of the Yangtze River urban agglomeration. At the same time, the CCD of the state subsystem and HQED was also relatively low. The relationship between the pressure subsystem and the response subsystem and the level of economic development is relatively close. It can be clearly seen from the radar chart that the higher the HQED level of cities such as Wuhan and Changsha, the higher the CCD of their HQED and the pressure subsystem and response subsystem.

Discussion and Recommendations
In recent years, the regional economy connected by urban agglomeration has developed rapidly, and the rapid economic development has exerted great pressure on the ecological environment [56]. The urban agglomeration in the middle reaches of the Yangtze River is the emerging growth pole of China's economy [57]. At the same time, the ecosys-

Discussion and Recommendations
In recent years, the regional economy connected by urban agglomeration has developed rapidly, and the rapid economic development has exerted great pressure on the ecological environment [56]. The urban agglomeration in the middle reaches of the Yangtze River is the emerging growth pole of China's economy [57]. At the same time, the ecosystem is complex and fragile. The coordination and co-construction of EE and HQED has become the primary issue for regional economic sustainable development. The existing research mainly focuses on the relationship between EE and urbanization. For example, Wu et al. examined the coupling relationship between urbanization and EE in Harbin Great Wall City Group; they observed that population urbanization has transformed into social urbanization and that the relationship with EE depicts a U-shaped curve [28]. Zou et al. evaluated the coupling coordination relationship and spatial heterogeneity characteristics of urbanization and EE in Shaanxi Province, and learnt that the development of ecological environment is lower than the speed of development of urbanization [29]. The coupling research on EE and HQED is relatively small. HQED is the inevitable choice of China's economy, moving from focusing on quantity to focusing on quality. Therefore, exploring the coupling and coordination relationship between EE and HQED was the focus of this study, with the hope of providing support for high-quality development and sustainable development of China's regional economy.
According to the evaluation results of the comprehensive evaluation index system of EE and HQED, the EE index and HQED index of the urban agglomeration in the middle reaches of the Yangtze River showed a synchronous growth trend during the sample period. However, from the perspective of the average value of the index of the 31 cities, the EE index and HQED index of all cities, with the exception of Changsha, showed significant differences. For example, the EE index of Ji'an and Fuzhou ranked first and second among the 31 cities, but the HQED index only ranked 23rd and 28th. Wuhan ranked first in the HQED index but 31st in the EE index. This feature conforms to the inverted U-shaped curve of environmental Kuznets [58], but the inflection point has not yet appeared. Economic growth and EE can be balanced, but the prerequisite is to implement effective environmental policies while ensuring economic growth. Therefore, environmental policies should be issued and implemented in a timely manner to promote the simultaneous development of the two. For example, Changsha has implemented the Xiangjiang River protection and treatment plan for many years, promoted the comprehensive treatment project of rivers, lakes and rivers, and strictly implemented the supervision and law enforcement work. EE and HQED have achieved synchronous development in the 31 cities.
From the analysis of the spatiotemporal evolution of the overall CD, CCD and interactive coordination relationship, it was found that EE and HQED have a significant coupling relationship and are at an advanced coupling level, but EE and HQED have not reached high-quality coordination, being always at the good and moderate levels of coordination during the sample period. Therefore, continuous attention should be paid to the improvement of the coordination level. Through the analysis of interaction and coordination, it was concluded that the innovative development and open development in the HQED subsystem, and the status subsystem in the EE subsystem, were the main factors that restricted the CCD of EE and HQED of urban agglomeration in the middle reaches of the Yangtze River. Coordinated development is the key proposition of regional economic development. Through CCD analysis, it was found that the coordinated development level of EE and HQED in the urban agglomeration in the middle reaches of the Yangtze River was low. For a long time, the urban agglomeration in the middle reaches of the Yangtze River has been one of the oldest and strongest industrial bases in China, relying on convenient land and water transportation and rich mineral resources, and has formed a biased industrial structure dominated by steel, building materials and automobiles. At the same time, there is the reality that the foundation of the tertiary industry is weak and the industrial technology is relatively backward, resulting in a low level of innovation development and an open development subsystem. This urban agglomeration is located in the middle reaches of the golden waterway of the Yangtze River, and is the key area for ecological protection of the Yangtze River Economic Belt. At present, the state subsystem of EE in the urban agglomeration in the middle reaches of the Yangtze River also lowers the coordination relationship.
Based on the above research, this paper puts forward the following policy recommendations.
First, adhere to the principle of ecological priority. Good EE is the fairest public product. Since China's reform and opening, rapid economic development has been at the cost of environmental pollution. EE is the inherent requirement of sustainable development of human civilization [59]. With an increasing income level, people's demand for high-level EE is increasingly strong. At the same time, the decline of human civilization is often caused by climate change, soil desertification and other factors [28]. The coordination between EE and HQED directly affects the ecosystem of the Yangtze River. Therefore, in view of the fact that EE lags behind in the development of HQED in certain regions, the principle of ecological priority should be followed.
Second, establish an ecological economic system. Chairman Jinping Xi proposed that lucid waters and lush mountains are invaluable assets. This concept highlights the economic value of ecology. High-level EE is conducive to gathering various economic resources, production factors and high-end talents, thus facilitating production development and technological innovation. The urban agglomeration in the middle reaches of the Yangtze River can rely on scientific and technological innovation to build a green technology system and expand the energy conservation and environmental protection industry and clean energy industry. At the same time, relying on the advantages of EE, ecological industries, ecological agriculture, ecological tourism and other ecological economies should be developed so that the green resources of the urban agglomeration in the middle reaches of the Yangtze River can be transformed into ecological products and services. The establishment of an ecological economic system can compensate for the lack of coordinated development and strengthen the coordination and cooperation between EE and various industries through the overarching and systematic characteristics of the system. Third, improve the evaluation system and institutional arrangement of HQED, and highlight the economic value of EE. The evaluation system is a baton, which integrates ecological benefits, resource consumption, environmental pollution and other indicators into an economic development evaluation system. It also integrates EE cost into the economic operation cost and establishes the pay-to-use system and ecological compensation system of intergenerational compensation resources, which is conducive to the rational use of ecological resources in the urban agglomeration in the middle reaches of the Yangtze River, and realizes the coupling and coordinated development of EE and HQED. At the same time, a perfect evaluation method can form horizontal comparisons and benign competition among cities, which is conducive to mutual learning among cities, and jointly achieving the coupling development of EE and HQED.

Limitations and Future Research Directions
Limited by the availability and continuity of data, some factors have not been included in the indicator system, resulting in certain limitations of the results. For example, the per capita total grain output of the status subsystem and the per capita industrial sulfur dioxide emissions of the pressure subsystem in the ecological indicators were not incorporated in the indicator system. However, if the time range of sample indicators can be expanded, the study will more clearly represent the coupling and coordinated evolution of EE and HQED in urban agglomeration in the middle reaches of the Yangtze River. Future research will further improve the coverage of the indicator system and expand the time range of the sample period.

Conclusions
EE and HQED restrict and promote each other. It is of great significance to study the coupling and coordination relationship between them for HQED and sustainable development. This study constructs a comprehensive evaluation index system of EE and HQED and measures the comprehensive level, CD, CCD, interaction and coordination of EE and HQED in the urban agglomeration in the middle reaches of the Yangtze River from 2010 to 2019, as well as the spatiotemporal change rules based on the evaluation results. These measurement results reveal that the EE and HQED of the urban agglomeration in the middle reaches of the Yangtze River have the following significant characteristics: (1) From 2010 to 2019, the overall levels of EE and HQED in the urban agglomeration in the middle reaches of the Yangtze River were improved simultaneously; however, there was significant heterogeneity in specific cities. (2) From 2010 to 2019, the EE and HQED of the urban agglomeration in the middle reaches of the Yangtze River had an advanced coupling relationship and a good/moderatecoordination level relationship. Therefore, improving the coordination between the two should be the focus in the future.